The present invention relates to a means for controlling the torque transmitted to a rigid steerable axle in a vehicle. More particularly, the invention controls the torque input to the steerable axle of a four-wheel drive (hereinafter, 4WD) front mount mower to minimize mismatched wheel speeds, skidding, and resulting damage to the turf. The advantages of this invention, however, are believed to be applicable to almost any vehicle driven by multiple axles.
A conventional vehicle drive train often includes a prime mover, such as an internal combustion engine, powering one or two wheels through a transmission and final drive axle. On off-road vehicles, like tractors and mowers, the combination of a hydrostatic transmission (hereinafter HST) and transaxle final drive is an alternative drive train configuration which is known to give greater variability of torque and vehicle speed than geared mechanical transmissions.
In many applications gradeability is a concern. Gradeability is the ability of the vehicle to traverse a grade or hill. Since the gradeability of a vehicle is generally limited by the tractive capability of its drive wheels, some vehicle makers have sought to increase gradeability by powering additional drive wheels. As a result, 4WD vehicles have become commonplace.
In a 4WD vehicle, the steering axle serves as a second source of drive power to enhance tractive capability. Power driven steerable axles are known and widely used on farm tractors. These tractors typically have a hydraulically operated clutch in the main transmission which selectively engages an external drive shaft to power the steerable axle wheels through a bevel gear and differential in the axle. Unfortunately, when the clutch is disengaged, the external drive shaft is still connected to the axle and wheels. Thus, the inertia of the external drive shaft can interfere with free rotation of the wheels and can cause them to skid.
Many 4WD cars and trucks have clutches to selectively supply power to secondary steerable axle wheels. However, such steerable axles are typically not rigid. These vehicles typically have external drive shafts extending from a center differential to independently suspended front wheel assemblies. Depending on the location of the clutch, such vehicles may also experience wheel skidding problems.
Because of their particular applications, wheel skidding is a big concern in front mount mowers. These mowers are frequently used for turf care at golf courses, country clubs, estates and the like. Where the unblemished appearance of the turf or grass is such a high priority, wheel skidding must be minimized. When the wheels skid they cease to roll freely, instead they slide across the ground. The tires tend to tear up the turf as they slide across it.
Traditionally, front mount mowers were two wheel drive vehicles. The drive wheel(s) were powered by an engine though a hydrostatic transmission connected to a front or primary transaxle. As the name indicated, the mowing deck was mounted at the front of the vehicle. Seeking better gradeability, some manufacturers have attempted to incorporate 4WD into their front mount mowers. In one 4WD system the steerable rear wheels are powered at a fixed ratio by the power takeoff (hereinafter, PTO) shaft of the same HST that drives the front wheels. As illustrated in FIG. 3, this presents a problem during turning of the mower.
As the highly maneuverable mower swings around a theoretical point of rotation O, due to the relative curvatures or radii of wheel paths A, B, C, D, the speed required of the outer rear wheel 10 is greater than the inner rear wheel 12 and both rear wheels must be going faster than the corresponding front wheels 14 and 16 to prevent skidding. The larger the radius of curvature for the path, the greater speed the wheel must travel to maintain that path. If the wheel is driven at a relatively constant speed and more speed is demanded to negotiate a turn, the wheel cannot keep pace and will slip or drag on the turf causing damage to the grass. Higher turn angles or sharper turns cause greater proportional mismatches in wheel speeds. Using spider gears in the axle is known to reduce inner/outer wheel speed mismatches, but front/back mismatches remain a persistent problem. If 4WD is to be achieved by powering the rear wheels at a fixed ratio relative to the front drive wheels, the resulting mismatch in actual versus required wheel speed during a turn will cause the rear wheels to skid and damage the turf.
If a clutch in the main transmission were used, the inertia of the required external drive shaft would contribute to wheel skidding and turf damage. Placing a device known in the prior art as a freewheeling or overrunning clutch between the drive shaft and the bevel pinion shaft of the steerable axle could reduce the inertia and skidding problem in the forward direction. However, because of the way those clutches operate, loss of tractive capability may still occur in the reverse and braking modes of operation.
Therefore, it is the primary objective of this invention to provide an electromagnetic clutch driven rigid steerable axle for a 4WD system which reduces wheel skidding during turns.
A further objective of this invention is to provide a steerable drive axle system which provides some driving power to the steerable wheels automatically when other wheels are driven, yet allows the steerable wheels to overrun through turns, and as otherwise needed, to avoid wheel skidding and turf damage.
A further objective of this invention is to provide a steerable drive system which allows the steerable wheels to overrun in both the forward and reverse directions.
A further objective of this invention is to provide a steerable drive system for a 4WD front mount mower, tractor or the like which reduces wheel skidding.
A further objective of this invention is to provide a clutch for a steerable drive system which transmits additional power to the steerable axle when commanded.
A further objective of this invention is to provide a steerable drive system wherein a sensor sends a signal representing HST system pressure to an electronic control means which actuates an electromagnetic clutch to provide additional grade ascending power to the steerable axle when a predetermined HST system pressure is exceeded.
These and other objectives will be apparent from the following description of the invention.